This invention relates to test and measurement instruments, and more particularly, to methods and apparatus for estimating cable length.
In the past, time domain reflectometry (TDR) has been used to determine cable lengths. However, the computations required for TDR are complex, and the TDR process requires a fair amount of power. Such power requirements are disadvantageous in certain situations, for example, when a battery powered/small size test instrument is desired. To overcome these issues with TDR measurement techniques, capacitance measurements have been used in test instruments to estimate the length of a cable under test. For example, it has been known to employ a sinusoidal excitation voltage and measurement of current to accomplish the measurement. However, in such a method, series resistance in the measurement circuits must be compensated for. When testing or measuring certain types of circuits, overload protection is necessary, to protect the instrument from damage in the event of connection with an excessive voltage. The overload protection adds series resistance that must be compensated for. Further, to accomplish the measurement, the frequency of the sinusoid must be known, and the voltage level must be controlled.
In accordance with the invention, cable lengths are determined by measurement of the capacitance of the cable. A square wave signal is applied to one conductor on the cable, and current is measured on the other conductor. The signal change gives the capacitance with enables determination of the cable length.
Accordingly, it is an object of the present invention to provide an improved capacitance measurement technique for determining cable length that does not require compensation for series resistance.
It is a further object of the present invention to provide an improved measurement technique for cable length that is adapted for portable or battery powered instruments.
It is yet another object of the present invention to provide an improved cable length measurement device that does not require compensation for measurement circuit resistance.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.